So, I decided to use javascript and change it width when the user resizes the browser's window. So, if my width screen is 1000px, it has to be 1000px. But when I resize the browser's window to 756px, it has to change to 756px via Javascript on the width="..." of fb:comments. The error is from Firebug (Firefox 8): box[0] is undefined -> var boxwidth = box[0].getAttribute("width");.